1. Field of the Invention
The present invention relates to magnetic recording media and, particularly, it relates to magnetic coating solutions having an excellent dispersibility and to high density magentic recording media having excellent surface properties, storage stability and a high reproduction output.
2. Description of the Prior Art
Hitherto, as magnetic recording media, for example, video tapes, etc., those which are produced by blending and dispersing a metal oxide powder such as .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4 or CrO.sub.2, etc., as a ferromagnetic material in a binder, applying the resulting dispersion to a nonmagnetic flexible support and drying the layer have been used.
Although the recording wavelength in these video tapes is about 2 .mu.m in cases of small-sized VTR's, recently use of even shorter recording wavelengths has been required so as to increase the recording density.
However, in magnetic recording media using a metal oxide, a high reproduction output cannot be obtained when the recording wavelength is short because they have a low residual magnetic flux density (Br) and a low coercive force (Hc).
Therefore, development of a process for obtaining magnetic recording media having a high Br and Hc has been investigated. The following processes for producing ferromagnetic metal powders are known.
(1) A process which comprises reducing an oxalate of a metal capable of producing a ferromagnetic material in a hydrogen stream at a high temperature (e.g., as disclosed in Japanese Patent Publication Nos. 11412/61, 22230/61, 14809/63, 8027/65, 14318/66, 22394/68 and 38417/72).
(2) A process which comprises reducing goethite or needle-like .gamma.-Fe.sub.2 O.sub.3 in a hydrogen stream at a high temperature (e.g., as disclosed in Japanese Patent Publication Nos. 3862/60, 20939/64 and 39477/72, U.S. Pat. Nos. 3,598,568, 3,607,220 and 3,681,018, British Pat. No. 1,192,167 and German Patent Application (DT-OS) No. 2,130,921).
(3) A process which comprises evaporating a ferromagnetic metal in an inert gas followed by condensation (e.g., as disclosed in Japanese Patent Publication Nos. 25620/71 and 27718/72 and Japanese Patent Application (OPI) Nos. 25662-25665/73 and 55400/73 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application")).
(4) A process which comprises decomposing a carbonyl compound of a metal capable of producing a ferromagnetic material (e.g., as disclosed in Japanese Patent Publication Nos. 128/63 and 3415/65 and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882).
(5) A process which comprises electrodepositing a ferromagnetic metal in mercury using a mercury cathode and separating the mercury therefrom by heating (e.g., as disclosed in Japanese Patent Publication Nos. 787/64, 15525/64 and 8123/65 and U.S. Pat. No. 3,156,650).
(6) A process which comprises reducing a salt of a metal capable of producing a ferromagnetic material in an aqueous solution with using a reducing agent (e.g., boron hydride compounds, hypophosphites or hydrazine, etc.) to obtain a ferromagnetic powder (e.g., as disclosed in Japanese Patent Publication Nos. 26555/63, 4567/66, 4769/66, 20116/68, 16052/72 and 41718/72, Japanese Patent Application (OPI) Nos. 1353/72 and 41718/72 and U.S. Pat. Nos. 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556, 3,663,318, 3,700,499, 3,943,012, 3,966,510, 4,007,072, 4,009,111 and 4,020,236).
Of the above-described processes, a finely divided powder having magnetic characteristics of a coercive force of more than 600 Oe and a maximum magnetization of more than 100 emu/g wherein several finely divided particles having a particle size of about 200 A are connected in a chain can be easily obtained by the low vacuum evaporation process (3). However, the resulting low vacuum evaporation process ferromagnetic metal powder has a high activity itself and often ignites forming an oxide having very poor magnetic characteristics when the powder is allowed to stand in the air. Accordingly, difficulties arise because the powder should be handled in a vacuum or in an inert gas until the powder is mixed with a binder or a solvent in order to produce a magnetic coating composition. Further, since the resulting magnetic coating composition has poor dispersibility, the saturation magnetic flux density of the magnetic recording medium obtained by coating this composition on a nonmagnetic support and drying the coating decreases due to oxidation of the magnetic material at high temperature and high humidity and, consequently, the magnetic recording medium has a poor storage stability. Therefore, a process which comprises deactivating the surface of the ferromagnetic metal by oxidation has been proposed. A process which comprises mixing the formed ferromagnetic metal powder with an organic solvent and oxidizing the surface layer by drying the mixture in the air to remove the organic solvent and a process which comprises mixing a formed ferromagnetic metal powder with an organic solvent, drying the mixture in an inert gas to remove the organic solvent and oxidizing the surface by supplying air slowly thereto are known, as described in U.S. Pat. Nos. 3,206,338 and 3,663,318, as surface oxidation stabilizing processings. However, when these processes are applied to the low vacuum evaporation process, the resulting magnetic coating composition has poor dispersibility and the storage stability of the magnetic recording medium obtained by coating such a composition on a nonmagnetic support and drying the coating is not sufficient.